1. Field of the Invention
The present invention relates to the art of papermaking and, more particularly, to the continuous determination of paper strength during manufacture of paper sheet materials.
2. State of the Art
In the papermaking industry, strength specifications are commercially important for numerous paper products including bag paper, liner board, corrugating medium, newsprint, and tissue paper. As a result of custom and developments over the years, strength specifications are usually based upon standardized laboratory procedures for determining properties such as burst strength, tensile strength, elongation, internal tearing resistance, edge tearing resistance, crush strength and so forth. A specific example of a widely accepted laboratory test is the Mullen burst test. A Mullen test is usually conducted by clamping a sample of paper across a ring and then providing a diaphragm to increase pressure against one side of the clamped paper until it bursts. The pressure at which the sample bursts is called the Mullen burst strength test. (Standard specifications for this test include TAPPI 403os-76 and ASTM D774.) Another example of a customary laboratory test procedure is the "STFI" compression test for heavy papers established by the Swedish Technical Forest Institute. In the STFI test, a sample strip is held between a pair of clamps that are moved towards each other while compressive force is monitored; the maximum compressive force is called the STFI compressive strength of the paper. (Standard specifications for this test include TAPPI 7818os-76 and ASTM D1164.)
Still another example of a widely accepted laboratory test is the standardized tensile strength test wherein a sample strip of paper is pulled in opposite directions with progressively increasing force until the sample fails; the tension at the failure point is called the tensile strength of the paper. (Standard specifications for this test include TAPPI Standard T404os-76 and ASTM Standard-D828.)
Laboratory test procedures in the papermaking art, however, have certain inherent limitations. One critical limitation is that substantial periods of time are required for sample acquisition and analysis. During these periods, production conditions may change sufficiently that the laboratory tests results, when available, are no longer representative of current manufacturing or product conditions. Another limitation is that almost all laboratory tests detect physical failure of paper materials and, thus, are necessarily destructive tests. Yet another limitation is that laboratory tests inherently involve sampling, and the relatively small samples obtained for testing may not completely or accurately represent sheet material that has been produced. Because of the above-mentioned limitations and the fact that paper quality laboratories can test only a small fraction of the paper produced by papermaking machines, it often happens that enormous quantities of substandard paper are produced before a quality laboratory discovers production problems.
In an apparent effort to automate laboratory test procedures, U.S. Pat. No. 4,550,613 suggests an apparatus for automatic determination of the tensile strength properties of a sheet of paper. The apparatus includes a cutter to cut a sample of paper of standard width and a device for measuring the tensile strength properties of the sample.
In light of the limitations of standardized laboratory procedures, whether automated or not, workers in the papermaking art have sought to make continuous measurements of paper strength on-line, i.e., while a sheet-making machine is operating. On-line measurements, if made rapidly and accurately, have the potential to enable nearly immediate control of papermaking processes and, thus, to substantially reduce the quantity of substandard paper that is produced before process conditions are corrected. In other words, on-line measurements have the potential to substantially reduce time delays between the occurrence and correction of "upset" conditions in papermaking processes. In practice, however, on-line measurements of papermaking processes are difficult to make accurately and often cannot be well correlated with standardized laboratory tests.
One of the difficulties in making accurate measurements of sheet material on papermaking machines arises from the fact that modern papermaking machines are large and operate at high speeds; for example, many papermaking machines can produce sheets up to four hundred inches wide at rates, called "wire speed," of about 20 to 100 feet per second. Another complication affecting on-line measurements is that physical properties of paper sheet material can vary across the width of a sheet and may be different in the machine direction than in the cross sheet direction. (Thus, in laboratory tests, paper strength typically has different values depending on whether test strips are cut in the machine direction or the cross direction.)
Because laboratory tests of paper sheet characteristics are normally destructive in nature, such test procedures cannot be readily adapted for obtaining on-line measurements. On the other hand, because commercial custom is such that laboratory tests of sheet properties are the yardstick for acceptability of on-line measurements, only on-line sensors whose outputs correlate well with laboratory tests of sheet properties are likely to have maximum acceptance in the papermaking industry.
One specific example of a suggestion to provide on-line measurement of mechanical properties of paper sheet materials appears in U.S. Pat. No. 4,291,577, assigned to the Institute of Paper Chemistry and entitled "On Line Ultrasonic Velocity Gauge." This patent describes a system for measuring velocities of ultrasound waves through traveling paper webs using a device having spaced-apart wheels that roll along a traveling paper web; the wheels have transducers on their peripheries to impart ultrasound signals to the web. According to the patent, output signals from the transducers can be utilized to measure the velocity of sonic waves through the web. Also the patentee suggests that the sonic velocity measurements can be correlated with Young's elastic modulus which, in turn, can be used to estimate paper strength. (See also Baum, G. A., "Paper Testing and End-Use Performance" printed in "Compressive Strength Development on the Paper Machine", Institute of Paper Chemistry, 5-8, 1984.)
Other workers in the art have also suggested that correlations exist between tensile strength, burst strength and sonic velocity through a paper web. See, "On-line Measurement of Strength Characteristics of a Moving Sheet: Ming T. Lu, TAPPI, 58(6):80 (June 1975). Also see Seth, R. S., and Page, D. H., "The Stress Strain Curve of Paper" in "The Role of Fundamental Research in Paper Making", PIRA Symposium Proceeding, Cambridge, 1981, wherein it is reported that the elastic modulus of a sheet relates to the elastic modulus of the fibers, the mean length and width of the fibers and the relative bonding area. Also see U.S. Pat. No. 4,574,634 that disclosed a device employing sonic transducers to detect the machine direction and cross-direction Young's moduli for paper samples. Further, in U.S. Pat. No. 4,335,603, assigned to Beloit Corporation, it has been suggested that tension in a moving paper web can be detected by measuring the time of travel of a sonic wave through the web.
By definition, Young's modulus indicates the rate of change of a stress-strain relationship. In the relationship as applied to paper materials, stress refers to loading force applied to a paper specimen and strain refers to elongation of the specimen in response to the applied force. It has been observed that, when Young's modulus is determined for a given specimen of paper, the failure point of other paper of the same kind can sometimes be predicted. In practice, however, Young's modulus has not been rigorously related to papermaking process conditions that affect paper strength and it is known that some processing steps may increase the strength of paper of a certain kind with little substantial change in Young's modulus and that other processing steps, such as wet straining, may substantially affect Young's modulus for certain kinds of paper with substantially less effect upon paper strength measures. See, for example, the article by Seth and Page, supra.
As further background to the present invention, it is useful to generally describe a typical papermaking process. Broadly speaking, a papermaking process begins when a slurry of fibers and water, called raw stock, is spread from a reservoir called a "head box" onto a wire mesh that supports the web while allowing substantial drainage. After the wet web of fibers is formed, the web is passed through a press section where water is squeezed from the web and then through a dryer section where water is evaporated from the web. After the dryer section, the web passes through calendar rollers to provide surface finish and then, usually, through a scanner and onto a reel. The portion of a papermaking process prior to a dryer is often referred to as the "wet end" of the process. It can be appreciated that on-line measurements at the wet end are desirable because such measurements, if acted upon promptly, can provide control early enough during paper production to allow process changes before substantial quantities of substandard paper are produced. On the other hand, wet end measurements are difficult to make because of the high water content of paper webs at this stage and because of frequently severe environmental conditions.
Still further as background to the present invention, it should be understood that papermaking machines have been instrumented to include sensors to detect parameters such as wire speed, basis weight, moisture content, and caliper of the paper during production. Many of the on-line sensors are designed to periodically traverse or "scan" traveling webs of sheet material to provide successive measurements across the webs. (In the sheet-making art, a succession of measurements at adjacent locations that, in total, spans a traveling web in the cross direction is called a "profile.") Scanning systems are advantageous because, as mentioned previously, various properties of paper may vary across a sheet as well as along the sheet; particularly, cross-direction strength properties may be different than machine direction strength properties.
Examples of scanning systems are provided in U.S. Pat. Nos. 3,641,349; 3,681,595; 3,757,122; and 3,886,036 assigned to Measurex Corporation. Other specific examples of scanning gauges proposed by workers in the art include ones that detect the composition of sheet material by measuring the radiation absorbed from beams of infrared light or other radiation of known wavelength directed against a given area of the sheet material. Devices of the latter type operate in accordance with the general principal that the amount of radiation absorbed by sheet material at a particular wavelength is a function of the composition of the material. Also, in U.S. Pat. No. 4,453,404 assigned to Mead Corporation, there is described a scanning system for determining statistical characteristics of sheet material; the patent states that the system can monitor the weight basis of sheet material, such as paper, as the material is being produced. Still further, in U.S. Pat. No. 2,806,373 there is disclosed an apparatus for testing sheet paper comprising at least two detectors that are continuously responsive to thickness and opacity variations. The patent states that, for paper produced in a given paper mill from given raw material, relationships exist between various characteristics of the paper and that a knowledge of some of the characteristics permits conclusions to be drawn regarding other characteristics; particularly, the patent states that variations in porosity and moisture can be obtained as algebraic functions of variations of thickness and substance.
Still further as to prior art, it may be noted that U.S. Pat. No. 3,687,802 describes a method and system for controlling the moisture content, mullen, and basis weight of paper by measuring each and developing appropriate control signals for adjusting a papermaking machine so that the desired measurements are approximated. Also, U.S. Pat. No. 3,936,665 discloses a sheet material monitoring apparatus including sensing gauges and a computer for determining a data profile across the sheet material. According to the patent, the monitored data may be used to provide information to control a sheet-making process to obtain a desired characteristic of the sheet material. Further, the patent teaches the data profile of a characteristic of a sheet is to be obtained without using scanning gauges.